Thermionic valve oscillator



i UNI ED STATE PATE E?" THERMIONIC VALVE oscitLA'ron Clifiord HenryJames Beaven, Woodmanster'ne, Kenneth Montague Caple, London, and-Hugh 1Alexander Dell; Banstead, England, assig nors to Hartford National Bankand Trust Gompany, Hartford, Conn as trustee "Application April 5, 1949,Serial No. 85,648 In Great Britain August 30, 1947 .1. Thisinventionrelates to thermionic valve oscillators of the resistance-capacity type.

It is an inherent feature of all low-frequency resistance-capacityoscillators which are tuned by means of a small variable condenser thatthe oscillator output point of the frequency discriminating network isof-high resistance and high reactance. This feature is a disadvantage inthat stray capacities lead-to reduced frequency coverage with a givenvariable condenser and'involve the use of a frequency scale which is notsimply calculable. I

In addition in coupling the oscillatoroutput point to a valve amplifierdifiiculty may arise in that the maximum permissible input resistance ofthe valve amplifier is exceeded. 1 The object of the present inventionis to provide a-circuit-arrangement in which the above difiiculties aresubstantially avoided. A further objectofthe present invention istoprovide .an oscillator the output of which is. stabilised in amplitude,and substantially independent of valve characteristics. -According tothe invention a thermionicvalve oscillator comprises a frequencydiscriminator network for determining the frequency of the generatedoscillations, the network having a pair of series connected arms soarranged that out-ofphase potentials are developed across the ends"of-.thenetwork, the potentials being of such magnitude relative toearth potential that. the junction'point of the arms, of the network hasa :potential which is low relative to the said out-of-.phase-potentials. According to a further feature of the invention -athermionic valve oscillator as set forth in the preceding paragraphcomprises means for applying negative feedbackto the oscillator .valvesuch that the amount of feedback is dependent on the amplitude of theoutput of the oscillator...

Further features of the inventionwill be apparent from the followingdescription which is given by way of example and with reference to theaccompanying drawings, in which:

Figure 1 shows a circuit diagram of a thermionic oscillator according tothe invention.

Figures 2 and 3 show modifications of the circuit of Figure l, and ffFigure 4 shows a practical embodiment of the =-i nvention'. 7 V

eferring now'to Figurel, Ziand Z2 arearms of a frequency discriminatingnetwork and an A. C. signal is supplied by the oscillator valve V2 whichis backcoupled to the anode of an amplifier valve V1. It is an inherentproperty of such a cir- 6 Claims. (01-. 250-36) I "cuit that :11 a gae].A. c.

the-stage gain of the valve V1.

signal is supplied between the anode of valve V2 and earth, a signalwill be developed between the anode of valve V1 and earth ofoppositesense and of a magnitude depending on the ratio of Z1 to Z2, ifthe gain of V1 is high. The signal at the anode of valve 4V1 will thenbe"v largely independent of the gain .of

valve.V1- and the potential above earth at the oscillator output pointbetween the junction of Z1 and Z2 will be small 1. e. approximatelyequal to the signal at the anode of valve V1 divided. by Preferably thenetwork Z2.is a parallel network since it is the lower impedance so thatassuming a given gainof valveV1; andia givenvoltage atthe anode of valveV2 the signal at the junction of Z1 and Z2 will be smaller; In addition;it is desirable to shunt the anode load of valveVr as little as possibleand the output; from the oscillator/amplifier circuit .is

therefore better: obtained from the anode of V2. 1 In acircuit-arrangement according to Figure'l it was found thatusing astandard two gang variable: airz'c'ondenser of the type normally. usedin commercial radioreceivers, a satisfactory fre,- quency coverage of atleast 20:1 was obtained whereas in existing circuits the ratio isrestricted touabout 10:1. The grid leak for the amplifier valve V1 maybeas low as l megohm although the resistors in the frequencydiscriminating network Z1 and Z2. are of the order of ten times thisvalue. .;In:order tostabilise the amplitude of the oscil; latorl outputindependently of valve character.- istics it is convenient .to provide acircuit-arrange ment such as that shown in Figure 2. In this arrangementcomprising valves V1 and V2, resistance's Ria'nd R4 and a lamp L1, theproportion of negative feedback applied to the grid of valve V2 is'dependent'on thej'a'mplitude' *of'the signal developed at its anodesince the resistance of the lamp L1 increases with applied signal..nz'Ifhe'resistors R3 and R4 may be replacedbyfre-.quencyqdiscriminatingnetworks Z1, Z2 as shown in Figure 3, in whichcase the inherent frequency stability and waveform is improved.practical; embodiment of the inventiorr is shown. in greater ,detail inFigure 4 in whichcir: cuit-arrangement the control characteristics arefurtherimproved by ensuring that only A C. is passed through thelamp'L1. 4 ....The pentodeoscillator valve V2 is of thegtype nectedthrough a resistor 2 of 47 k. ohms to the H. T. positive line 3. Theusual voltage stabilising capacitor 4 of 32 ,ILF. is connected betweenthe screen of the H. T. negative line 5. The cathode (and suppressorgrid) .is connected through self bias resistor 6 (150 ohms) to the line5. j

The control grid is connected through grid resistor '1 and lamp 8 (230volts, 15 watts) to the line 5 and the junction of this resistor withthe lamp is connected through resistor-9 of 5 k. ohms to one pole ofcapacitor I (32 ,uF.) .the other pole of which is connected to the anodeof V2. The output of the oscillator is taken from-thejunction ofresistor 9 and capacitor I40, .the output appearing across the slider ofpotentiometer. k. ohms) and line 5 (terminals. A), the potentiometerbeing connected through theresistor l2 of 6.8 k. ohms to the resistor 9.

The control grid of valve V2 is also connected through resistor l3(shunted by preset capacitor 114) .and'capacitor l5 of 0.5 -,uF. to.theanodeof .walve V1 which is in turn connected through load resistorL6 to the H. T..line 3. Theascreen grid of valve V1 is energisedthroughresistor I! of 220 k. ohms from line 3 andthe usual capacitor J0 (0.5F.) is included between this grid'and linel5. Thecathode of valve V1 isjoinedto line i through.biasresistor. [9 (1.5 k. ohms) by-passedbycapacitor .20 (2.5 1F). A grid resistor 2| of 3.3 m..ohms is connectedbetween the control grid .of V1 and line 5,.the grid beingalso connectedthrough capacitor .24 (0.22 F.) with the junction of the impedancenetworkszrand Z2 previously described.

The network.Z1 comprises resistor R1 having one-end connected to theanode V2 andonesec- 'tion 22 of a normal 2-gang variable capacitor(14-470 pF.) as commonly used in radio receivers, to the fixed vanes ofwhich;.the other end of resistor R1 is joined. The network Z2 comprisesthe second section '23 ofthe .gangcapacitor with resistor R2 inparallel. The section -23 .has its fixed-vanes connected to theanodeofvalve V1, the moving :vanes of both capacitor :sections 22 and 23being connected to .the control grid of valve V1 through the capacitor:24. Variable '(preset) trimmer capacitors of .3-30 .pF. are shuntedacross 7 each section 22, :23.

'The above I described oscillator is adapted 1150 cover the range offrequencies extending from '30 C. P. S. to 100,000 C. P. S. in threeranges depending on'the values of resistors R1 and RziflS shown in thefollowing table:

R1 and R2 Frequency range 30-600 0. P. S. EGO-10,0000. P; I5,000-100,000.C.

S. P. S.

If desired, switching arrangements may be provided to bring in therequired resistors to enable be made to suit any particularcircumstances as they arisein practice. It will be clear that thecircuit arrangements shownin Figures-2'and *3 will also show improvedperformance if the lamp L1 is parallel-fedas shown in Figure'4.

4 We claim: 1. A resistance-capacitance oscillation generator comprisingan oscillator stage provided with a first electron discharge tube havinga cathode.

.a gridandananode, anamplifier stageprovided .-With asecond electrondischarge tube having a cathode, a grid and an anode, afrequency-discriminating network connected between the anodes of saidfirst and second tubes and constituted by-first and secondserially-connected impedance arms, means to couple the junction of saidarms tothe grid of said second tube, and means regeneratively to couplethe anode of said second tube to .the grid of said first tube tosusttain oscillations in said oscillator stage, said second arm havingan impedance value with respect to said first arm at which the magnitudeof voltage developed at said junction relative to ground is small ascompared to that developed at the anode of said second tube.

2. .An arrangement as set forth in claim 1 further including a negativefeedback circuit coupled between the anode and grid of .said first tubeto provide feedback to an extent depending on the amplitude of theoscillation outputof the oscillator stage.

with a resistance, said second arm being, formed by a condenser inparallel with .a resistance,

means coupling the junctionofsaid arms toxthe grid of the second tube,and means regenerative- .ly coupling the anode: of. said: secondvtubeto'tl'le grid of the first tube.

4. An arrangement as set forth in .claim :3 wherein the condensers insaidsfirst andsecond arms form individual sections .of a variable,

ganged capacitor.

5. An arrangement as set forth in claim wherein the resistancesin-saidfirstandsecond arms are of like value.

6. An arrangement .as set forth .inclaim. :3

further including a. negative feedback circuit-in said oscillator stage:provided :withan' incandescentlamp connectedlin the ano'de circuitofzsaid first tube to develop a feedback voltagedependinz .on'theintensity of oscillations produced insaid stage, and meansrtoapplysaid'feedback voltage to the gridzof said first tube.

CLIFFORD HENRY JAMES xBEAVEN. KENNETH MONTAGUE-CABBE. HUGH ALEXANDER.DELL.

REFERENGES' CITED The following references are of record inthe file ofthis'patent:

UNITED STATES PATENTS Number Name Date 2,268,872 Hewlett .Jan. 6, 19422,303,862 Peterson Dec. 1, 1942 2,346,396 Rider Apr. 11,1944 2,444,084Artzt June=29,.1948

